1. Field
The disclosure relates to a tire for civil engineering vehicle (of the type called “off-road” in English) having a tread with a thickness of at least 25 mm. More specifically, the disclosure relates to a new type of tread having a sculpture pattern capable of limiting the differences in shear strain between the tread pattern elements that may be created, notably, by running along a curved path.
These differences in shear strain may give rise to an embrittlement of the bottoms of the grooves forming the tread pattern.
2. Description of Related Art
A tire intended to be fitted to a civil engineering vehicle is usually provided with a tread which radially surmounts the outside of a crown reinforcement which itself surmounts a carcass reinforcement. This tread has a pattern formed by relief elements (ribs or blocks), these relief elements being delimited by a plurality of grooves; the thickness of this pattern is at least 25 mm and may be as much as 110 mm (“thickness of the pattern”, in this context, means the maximum depth of the grooves).
The relief elements of the tread have faces—called contact faces—located radially outside the tread and provided for making contact with the ground. The grooves have a depth equal to or less than the thickness of the tread, and have geometries, viewed in cross section, which are suitable for limiting the retention of pebbles and other objects present on the ground.
In running along a curved path, a considerable extension of the material forming the tread is observed, this extension being caused directly by the drifting of the tire and indirectly by the compression of the material forming the tread under the load supported, this load generating extension constraints by the Poisson effect. Owing to the position of each circumferential row relative to the centre of curvature during running along a curved path, a local variation of the distances travelled by each circumferential row is observed, and is manifested in differences in stress and shear between the rows.
These differences in stress and shear are further amplified by present-day requirements regarding tires for civil engineering vehicles, leading to an increase in the load-carrying capacity. In these operating conditions, there is a demand for improved traction performance on different types of ground and, as far as possible, improved wear performance (increased distance travelled before removal of the tire for a given wearable thickness).
In order to meet these requirements, there is a known way of increasing, to a very substantial degree, the thickness of wearable tread material and consequently increasing the groove depths, thereby increasing the heights of the relief elements.
During their passage through the contact patch, the relief elements are subjected to compressive forces and tangential stresses which bend them in a circumferential direction, in a transverse direction, and also in an oblique direction. These movements, which increase with the height of the blocks, may give rise to rapid localized wear as well as cracking of the rubber on the bottoms of grooves, which may then lead to infiltrations of water as far as the inner reinforcements of the tire.
To limit this phenomenon, there is a known way of placing platforms between the blocks, the purpose of these blocks being to limit the bending movement of the blocks during running. These platforms extend over only a part of the height of the blocks, so as to retain the greatest possible edge length, at least in the new state. A drawback of the presence of these platforms is the reduction in the available groove volume, as well as the reduction in the active edge length when the tread is worn down to the level of these platforms.